The rocks of the Blue Ridge physiographic province bear witness to the power of large scale volcanism, metamorphism, and erosion. They are the oldest rocks in the National Capital Region and form the “basement” for all the other rocks.
The Blue Ridge province was once part of an ancient supercontinent that existed 1 billion years ago in the Proterozoic Eon. Approximately 600 million years ago, rifting began to pull this supercontinent apart.
The rift created a basin for a new ocean which existed long before the Atlantic, the Iapetus Ocean. It also left behind igneous rocks, which were formed when magma rising from deep within the Earth erupted on the surface. This is now the most common type of rock of the Blue Ridge province.
As the Iapetus Ocean grew larger and the land masses spread farther apart, river and marine sediments were deposited on top of the igneous rocks. Eventually the Iapetus Ocean began to close, and three different mountain-building episodes occurred when land masses collided into the ancient North American continent.
These collisions metamorphosed, folded, and uplifted the previously deposited igneous and sedimentary rocks. The result was the Appalachian Mountains, which at that time soared to heights resembling the modern Himalayan range, which is also forming by collisions between two large land masses.
Erosion during the following 265 million years cut down the once towering Appalachian Mountains to modern elevations, and exposed the metamorphosed igneous and sedimentary rocks that were once buried deep within the core of the mountain belt.
You can stand on the ancient rocks associated with Iapetus Ocean rifting on the tall ridges of Catoctin Mountain Park. Metamorphosed sedimentary rocks (originally formed on the margin of the continent as the rift widened) now form the heights surrounding Harpers Ferry National Historical Park.
